An 8.6-magnitude earthquake struck below the ocean floor off the northern coast of Sumatra in Indonesia on Wednesday, triggering tsunami warnings across the Indian Ocean and causing families in the area to flee their homes in fear and confusion.

The quake, which was felt as far away as India and Sri Lanka, was originally reported as magnitude 8.9 before being downgraded by the U.S. Geological Survey (USGS) to an 8.6. Strong aftershocks were felt in the hours after the quake, including one registering a magnitude of 8.2.

Panicked Aceh residents evacuated buildings and tried to flee the cost by car, causing huge traffic jams. The sounds of “sirens and and Koran recitals from mosques” could be heard everywhere, said Sutopo, an Indonesian Disaster Mitigation Agency spokesman. On the Thai tourist island of Phuket, authorities blared tsunami warning sirens which had been set up in the wake of the devastating Boxing Day earthquake and tsunami in 2004. The international airport was also immediately shut down. Elsewhere, residents were evacuated to higher ground in six other Thai provinces.

High-rise buildings in the Indian city of Calcutta shook in a “minute of panic,” as one man described it in an interview with the BBC. “There was a tremor felt by all of us working in the building… all started ringing up to their family and asking about their well-being.” Maria Teresa Pizarro, a tourist in the Sri Lankan capital of Colombo, said she “could hear the wood in the furniture cracking, the curtains were moving and the ceiling fan was rattling. I just picked up the children and ran downstairs.”

The earthquake struck 20 miles under the sea floor about 300 miles from Banda Aceh, the capital of Aceh province. Its epicenter was only 125 miles away from the spot where the 9.1-magnitude quake struck on Dec. 26, 2004, causing a tsunami that swept away coastal communities in Indonesia, Thailand, Sri Lanka and India. About 230,000 people perished overall.

However, experts said after Wednesday’s quake that the likelihood of a similar tsunami was slim. Bruce Presgrave, a USGS official, told the BBC that a tsunami of that size was unlikely due to the fact the quake moved the earth horizontally rather than vertically: “We can’t rule out the possibility, but horizontal motion is less likely to produce a destructive tsunami.”

Susilo Bambang Yudhoyono, the Indonesian president, also attempted to reassure his people that there was no immediate danger of a tsunami, describing the situation in Aceh to visiting British Prime Minister David Cameron as “under control,” and that there was merely “a little bit of panic.”